Various types of dot matrix line printers have been proposed and are in use. In general, dot matrix line printers include a plurality of dot forming elements having their dot printing ends lying along a line that is orthogonal to the direction of paper movement through the printer. Since paper movement is normally vertical, the dot printing ends usually lie along a horizontal line. Located on the side of the paper remote from the dot forming elements is a platen and located between the dot forming elements and the paper is a ribbon. During printing, the dot forming elements are oscillated back and forth along the horizontal line that they define. At predetermined positions, as required by the image or characters to be formed, the dot forming elements are actuated to create a dot. A series of thusly formed dot rows creates a row of characters. The paper, of course, is incremented forwardly at the end of each dot row. While the present invention was developed for use in dot matrix line printers to shuttle or oscillate the dot forming elements and, thus, finds its primary use in this area, it is to be understood that the invention can be used to shuttle carriages of other mechanisms, including other printer mechanisms, particularly those mechanisms requiring or desiring constant velocity shuttling.
In general dot matrix line printers fall into two categories. In the first category are dot matrix line printers wherein only the dot forming elements are oscillated. In the second catagory are dot matrix line printers wherein the actuating mechanism as well as the dot forming elements are oscillated. Regardless of the type, the portion of the printing mechanism to be oscillated is mounted on a carriage and the carriage is oscillated by a shuttling mechanism. Since the present invention is directed to carriage shuttling mechanisms, it is useful with both catagories of dot matrix line printers.
In the past, various types of carriage shuttling mechanisms have been proposed for use in dot matrix line printers. One such type of carriage shuttling mechanism includes a stepping motor that is actuated to cause step increments of carriage movement. At the end of each step, the appropriate actuators are energized to create dots. Thereafter, the carriage is stepped and the appropriate actuators energized to create further dots. Bi-directional movement is provided by stepping the crriage first in one direction and then in the opposite direction.
One major problem attendant to the use of stepping motors in dot matrix line printers is the speed limitation that they place on the rate of printing. As a result of this limitation, attempts have been made to utilize other types of motors, such as constant speed DC motors. One of the problems with the use of constant speed DC motors is that the prior art mechanisms for coupling the shafts of such motors to the printer carriages have produced variable carriage velocity. As a result, prior art carriage shuttling mechanisms using constant speed motors require that the position of the carriage be constantly determined and the position information utilized to synchronize the energization of the print element actuators. If such synchronization is not provided, precise position repeatability of print element actuators is not provided, whereby printed characters and images are distorted and/or blurred. Distorted and/or blurred images, of course, are unacceptable in environments where high quality printing is required or desired. More specifically, in order to produce high quality printing, it is necessary for a dot matrix line printer to be able to precisely position dots at the same position in each dot line. If this result cannot be accomplished, the resulting images and characters are blurred and/or distorted in some other manner. Stepping motor carriage movement mechanisms, of course, provide the desired dot position repeatability. However, as noted above, such carriage shuttling mechanisms are slow and, therefore, undesirable. While movement mechanisms utilizing constant speed DC motors (or other constant speed motors) are substantially faster, they have the disadvantage of producing a nonlinear carriage displacement versus time curve (albeit oscillatory), whereby relatively expensive position sensing and/or control circuits are required in order to precisely control dot position. In some systems a precise position sensor is required. In other systems (where the shape of the displacement versus time curve is known and repeatable) a position sensor is not needed. In the latter case, the electronic timing for dot printing can be varied to accommodate the variable dot-to-dot displacement timing, provided the printing dynamics allow dot printing at the shortest dot-to-dot times available. In other words the shortest dot-to-dot displacement time, based on printing dynamics, limits the speed of such systems during the longer dot-to-dot periods as well as the short dot-to-dot periods. As a result overall speed is less than it could be if dot-to-dot displacement time were constant. Obviously, it would be desirable to provide a carriage shuttling mechanism that is substantially faster than stepping motor type carriage shuttling mechanisms yet does not require relatively expensive position sensing mechanisms and control systems adapted to compensate for variable carriage movement velocity.
Therefore, it is an object of this invention to provide new and improved carriage shuttling mechanisms.
It is also an object of this invention to provide new and improved carriage shuttling mechanisms for dot matrix line printers.
It is another object of this invention to provide new and improved carriage shuttling mechanisms for dot matrix line printers that provide constant velocity carriage shuttling movement in the region where dots are to be printed.
It is a still further object of this invention to provide bi-directional, constant velocity, carriage shuttling mechanisms for dot matrix line printers.